I was looking for some advice on sizing ceramic tiles for cyanotypes and came across Photo Engineerīs APUG recipe:

10 g 250 BI photo grade gelatin
90 g Distilled Water at RT

Mix and gradually raise to 40 deg C.

Add 5 ml of 10% Chrome Alum solution.

Add a suitable wetting agent if needed if tests show problems coating.

Spread on clean, grease free tile at about 10 - 15 ml / foot square.

Allow to cure for about 1 week.

Has anyone tried this successfully? Is it possible to do the same using some kind of varnish instead?